The preparation of tooth cavities for acceptance of filling materials normally is a painful experience unless pain-relieving anesthetic drugs, such as Lidocaine injections, or nitrous oxide gas inhalation is first applied prior to the dental preparation. In conjunction with cavity preparation, it is customary for a dental assistant to spray a jet of water onto the tooth being cut by the bur of the handpiece and this frequently clouds or fogs the area being prepared so that the dentist has difficulty seeing where the cutting is occurring. Many modern handpieces operated at speeds up to 400,000 rpm in order that heavy cutting pressure is not required. However, at such speeds, heat can be generated up to 450.degree. F. with a bur pressure of 32 grams, for example, at 170,000 rpm.
It has been found that a minimum water flow rate of 16 cc/minute is necessary to form a stream with force enough to reach the bur tip area in a commercial handpiece having as many as four or five jets of flushing water. This situation is responsible for such aforementioned fogging of the dental operation site.
Current types of handpieces which have axial through passages present difficulties when endeavoring to restrict cooling liquid by such liquid tending to enter the head and bearings of the turbine and flushes the lubricant thereof.
Another difficulty with such aforementioned through passages in burs in a handpiece is some community water supplies carry impurities and chemicals that may be harmful to patients and can clog the passage in the bur. Also, when using cooling sprays of water, when the dentist stops the flow water, there is a tendency for a suck-back or retraction function to occur in which contaminants in a patient's mouth may be drawn back into the head of the handpiece where they remain and may even migrate into the fluid supply conduits or hoses. Hence, when used with the next patient, any such contaminants can be discharged into said next patient's mouth.